The present disclosure relates generally to downhole systems and methods for gathering data from subterranean formations. More particularly, the present disclosure relates to downhole systems having laser devices that are configured or designed for high-temperature operations, within a borehole, at temperatures in excess of about 115 degrees Celsius.
Logging and monitoring boreholes has been done for many years to enhance and observe recovery of oil and gas deposits. In the logging of boreholes, one method of making measurements underground includes attaching one or more tools to a wireline connected to a surface system. The tools are then lowered into a borehole by the wireline and drawn back to the surface (“logged”) through the borehole while taking measurements. The wireline is usually an electrical conducting cable with limited data transmission capability. Similarly, permanent monitoring systems are established with permanent sensors that are also generally attached to an electrical cable.
Demand for higher data transmission rates for wireline logging tools and permanent monitoring systems is growing rapidly because of higher resolution sensors, faster logging speeds, and additional tools available for a single wireline string. Although current electronic telemetry systems have evolved, increasing the data transmission rates from about 500 kbps (kilobits per second) to 2 Mbps (megabits per second) over the last decade, data transmission rates for electronic telemetry systems are lagging behind the capabilities of the higher resolution sensors. In fact, for some combinations of acoustic/imagining tools used with traditional logging tools, the desired data transmission rate is more than 4 Mbps.
In addition, while higher data transmission rates are desirable, many tools in current use would have to be completely reworked or replaced to incorporate new data transmission technologies. It would be desirable to facilitate faster data transmission rates with minimal changes to existing tools and equipment.
One technology that has been investigated for increased data transmission rates is optical communication. Optical transmission rates can be significantly higher than electronic transmission rates. However, even if fiber optic cables are used for data transmission, the issue of operating the downhole sensors and electronics in downhole high-temperature environments remains. The downhole sensors and/or electronics often are required to operate for extended periods of time at temperatures in excess of about 115 degrees Celsius and sometimes in excess of about 200 degrees Celsius.
Some sensors of a permanent system are often deployed with a monitoring tool that extends downhole and is integrally attached to the borehole casing. The attachment is typically accomplished with a mechanical surface force clamping device and the sensors are typically housed in a side passageway or lateral extending section associated with the sensor housing or production tubing which is laterally displaced from the primary flow passageway through the production tubing.
Many monitoring tools for permanently deploying seismic sensor arrays downhole are single level monitoring tools. However, due to the complex subsurface formation and strata and the various levels of the multiple production zones and reservoirs, multilevel monitoring tools are also required to monitor various levels simultaneously. The monitoring tool that deploys the sensor arrays will typically include a plurality of sensor housings or shuttles where each shuttle contains at least one sensor.
Similarly, in the area of borehole logging, the number of transmitters and receivers and the distance between transmitters and receivers has been increasing to improve the ability to detect formation characteristics in the undisturbed formation farther from the borehole. One method to get deeper penetration is to increase the distance between source and receivers, such that the receivers are detecting signals that are returned from further distances in the borehole.
Furthermore, oilfield application of fiber optics sensors has been progressing in recent years for monitoring of certain parameters. However, many downhole applications require high temperature operations, and optical devices such as laser degrade rapidly or do not operate properly at high temperatures. Therefore, use of fiber optics for communication between surface systems and downhole tools, as well as use of downhole sensors, in high-temperature conditions, within a borehole, has been limited.
In certain embodiments, the present disclosure proposes efficient and reliable methods and systems for transmitting data from a downhole tool at high temperature using optical fibers. In this, the methods and systems disclosed herein provide downhole laser sources that are suitable for high-temperature applications. One of the problems addressed herein is that of a downhole source. Although semiconductor lasers are utilized as sources for optical communications, the temperature range at which such sources can operate is limited, in particular, when the sources operate on a continuous basis. Most other types of laser cannot be modulated directly through their supply current, as can semiconductor lasers.
Accordingly, it will be appreciated that there exists a desire to improve upon conventional downhole methods and systems in order to improve sensing of downhole parameters and the transmission of downhole data.
The limitations of conventional designs noted in the preceding are not intended to be exhaustive but rather are among many which may reduce the effectiveness of previously known telemetry mechanisms. The above should be sufficient, however, to demonstrate that downhole data telemetry structures existing in the past will admit to worthwhile improvement.